DEGEADATION. 191 



graphic features. Deep river valleys and mountain gorges are everywhere 

 seen to have been produced by this agency, and the power of the streams in 

 carving their winding paths is readily comprehended. But the magnitude 

 of this agency is more thoroughly brought into visual comprehension in the 

 caiions that traverse the Plateau Province, for here erosion has not kept pace 

 with corrasion. All the processes of erosion and sapping serve to obliterate 

 the evidences of corrasion, and the latter appears more plainly as its pro- 

 gress exceeds that of the other methods; but still the evidences of corrasion 

 rarely disappear until the land is buried by the sea; for wherever an area of 

 land is above its base level of degradation, there corrasion will be manifest 

 by deepening its channel; and wherever the dry land has been brought 

 down near to its base level, there corrasion is manifest by widening its 

 channel. 



There is another method of degradation to be considered, viz : 

 SAPPING. The walls that inclose the channels of corrasion are broken 

 down by gravity, and when in the progress of corrasion the channel of a 

 stream reaches beds which easily disintegrate, having passed through beds 

 which disintegrate but slowly, degradation is increased by an undermining 

 process, and as corrasion still continues through a series of yielding and 

 unyielding beds, the walls of the streams are carried back in a series of steps, 

 the tread of each step being the summit of a harder bed, the rise of each step 

 the escarped edge of the harder bed above, underlaid by the softer. It is 

 manifest that the conditions favorable to the continuatiorh of this cliff degra- 

 dation to any great distance back from the stream are found only where the 

 beds are horizontally, or nearly horizontally stratified. But sapping is not 

 confined to the undermining of walls produced by corrasion, but is carried 

 on in simple anticlinal upheavals from the axis toward the flanks; in up- 

 heavals of the Uinta type, in like manner, and in the blocks displaced as 

 integers, like those in regions having the Kaibab and other structures, from 

 the elevated to the depressed portions. In these cases the cliffs are produced 

 by the unequal erosion of harder and softer beds wherever upheaval exceeds 

 degradation, and climatic conditions are favorable; and, further, this sap- 

 ping process is carried on in regions of great declivity, where deep channels 

 of corrasion are formed, whatever may be the petrologic conditions, the 



